1. Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor, and more particularly to a method of manufacturing a thin film transistor using a polysilicon layer as a channel layer.
2. Description of the Related Art
In general, an active matrix type-liquid crystal display(AM-LCD) device is thin, so that it is often used in various display devices. In this AM-LCD device, one TFT is provided as a switching element for each pixel, so that individual pixel electrodes are independently driven.
In this display device, a glass substrate is used for a substrate material, for reducing cost. In this case, process must be performed at the low temperature of about 600.degree. C., considering the softening temperature of the glass. Therefore, in case a polysilicon layer is used as a channel layer, an amorphous silicon layer is crystallized by a laser annealing capable of thermally treating at the low temperature of 600.degree. C. or less, thereby forming the polysilicon layer. In this laser annealing, since fever is momently transmitted to the surface of the amorphous silicon layer, not to have on effect on the glass substrate, it is possible to crystallize the amorphous silicon layer without occurring the transformation of the substrate.
However, when crystallizing the amorphous silicon layer, a liquid state and a solid state coexist therein, so that the grain boundary of the polysilicon layer is projected due to the density difference of the two states, thereby generating hillock on the surface of the polysilicon layer. As a result, the surface of the polysilicon layer becomes rough, thereby deteriorating the properties of the TFT. For example, electric field is locally concentrated on the channel so layer of the TFT and leakage current occurs. Furthermore, the property of a gate insulating layer formed on the polysilicon layer is deteriorated. Accordingly, the reliability and the reproductivity are deteriorated.